A sealed envelope based on a filamentary winding of glass fibers impregnated with resin is described, for example, in French patent number 2 619 244 protecting a lightning arrester (c.f. U.S. Pat. No. 4,864,456). According to that patent, a stack is initially built up from pellets of a varistor type material, spacers, end fittings, and means for ensuring electrical continuity both between the pellets themselves and between the pellets and the end fittings; a filamentary winding of resin-impregnated glass fibers is then formed around the assembly so as to ensure bonding and radial binding of said stack without significant longitudinal compression. After the resin of the envelope formed in this way has set, a coating of EPDM type elastomer material is injected thereover.
The function of the winding is to hold the pellets together by having a high degree of adhesion on their side surfaces and having very high mechanical strength in the radial direction.
It may happen that an internal short circuit of several kilo-amps is initiated from pellets that are electrically weakened or partially short-circuited.
This gives rise to excessive heating and to a very large increase in internal pressure.
Regardless of circumstances, safety reasons require that no part of the equipment can be expelled from the envelope.
However, there can be no question of installing valves which would be far too complex for equipment considered as "consumable".
The object of the present invention is to propose a type of envelope suitable for solving this problem.